The early receptor potential (ERP) of hyperpolarizing photoreceptors in the scallop retina has been investigated. The ERP can be elicited with bright white light and is typically biphasic. It can be isolated from the late receptor potential by removal of external Na ion and by hyperpolarizing the membrane to the Kion equilibrium potential. The ERP can be fractionated into two component, a negative going component elicited by short wave length light and a positive going component elicited by long wave light. The spectral sensitivity of the negative component is the same as the spectral sensitivity of the late receptor potential in the dark adapted retina and corresponds to the "rhodopsin" state with a maximum of about 500 nm. In contrast, the spectral sensitivity of the positive component of the ERP has a maximum of about 575 nm and corresponds to the "metarhodopsin" state. The two components of the ERP are thermally stable and interconvertible. Our results show that photoisomerization of rhodopsin to metarhodopsin is associated with an increase in Kion conductance and that photoregeneration of rhodopsin from metarhodopsin is associated with a decrease in this conductance.